Sounds like the left/right Item movement might have been accidentally locked.
Lock settings are accessible via the Action List (Options: Show lock settings) or via Right-Click on the Lock-Button in the Main Toolbar.
More info here:
https://www.reaper.fm/videos.php#0kMMdgXz5gM
And welcome to the Reaper Forums!
In case you haven't already found them, make sure to check out the User Guide (
https://www.reaper.fm/userguide.php) and Videos section (
https://www.reaper.fm/videos.php) in general.
Lots of useful info there for getting started.